You may see the effect of light which comes only from one side on plants grown in the windows of rooms. If they are left in one position they grow in a one-sided manner with only the bare stalks toward the darker side of the room and all their leaves turned towards the window through which the light comes. If you want them to look pretty towards the room side also, they must be turned round frequently, so that the leaves are drawn in many directions instead of one only. The usual effect of light is to make the leaves grow towards it. You may see this still more clearly by placing a pot of seedlings in a blackened box with a small hole on one side. Very soon they will bend over towards the light entering by it (~see~ fig. 23). [Illustration: Fig. 23. Grass seedlings growing in an earthenware dish enclosed in a strong box blacked inside so that the light only enters at ~a.~ (Note how the seedlings bend towards it.)] Leaves can absorb most light when their upper surfaces are at right angles to it, and you will find some leaf-stems will bend right round in order to allow their leaves to get into this position. For example, if you take a pot of nasturtiums growing in the usual way, and support the pot on a stand, and cover it over with a bell jar which has been blackened, or with a black box, so that all the light reaches the plant from below, you will find that in a day or two the leaves will have turned completely round on their stalks and are now facing the light, so that they are upside down in their relation to the position of the whole plant (~see~ fig. 24). [Illustration: Fig. 24. Nasturtium covered over, so that the light only enters from below. The leaf surfaces bend over to face it.] [Illustration: Fig. 25. Spray of Maple showing stalks of leaves of the same pair of very different lengths, so as to place the leaves well as regards light.]